Mass spectrometry



June 8, 1954 c. E. BERRY MASS SPECTROMETRY 2 Sheets-Sheet 1 Filed Dec.5, 1951 FIG.

ZERO LEVEL c 0 I/VCREAS/NG MASS FIG. 2.

T0 EVACUAT/NG SYSTEM /ON SOURCE C //?C U/ T INVENTOR. CLIFFORD E. BERRYATTORNEY June 8, 1954 C. E. BERRY 2,630,312

MASS SPECTROMETRY Filed Dec. 3, 1951 2 Sheets-Sheet 2 I N V EN TOR. c4IFFORD E. BERR r ATTORNEY SENS/NG CIRCUIT i 4 3 5 5 7 m n NC m 0 4 a 5 f4 2 4 m GT F w/ a w u. 5 mm SC L n 0 mm 5 BMT 6 c P .w m F 5mm PatentedJune 8, 1954 MASS SPECTROMETRY Clifford E. Berry, Altadena, Califl,assignor to Consolidated Engineering Corporation, Pasadena, Caliil, acorporation of California Application December 3, 1951, Serial No.259,584

Claims. 1

This invention relates to mass spectrometry and particularly toimprovements in apparatus whereby undesirable effects or negative ionsare minimized or eliminated.

In mass spectrometry a sample to be analyzed is first ionized as bybombardment with an electron beam. Positive ions are thus produced andare collimated into a heterogeneous ion beam which is propelled into theinfluence of suitable electric or magnetic fields. These fields act uponthe beam to disperse it into a plurality of spatially separatedhomogeneous beams, each of which is composed of ions of a givenmass-tocharge ratio, 1. e. specific mass. One or more of theseindividual beams is selectively focused on and discharged at a collectorelectrode, the resultant discharge current constituting a measure of theabundance of the ions of a given specific mass.

In the process of directing these ion beams from the point of origintothe point of collection, positive ions, particularly those not infocus, are caused to strike and discharge upon confining metal surfaces.In this process negative ions of considerable energy are produced at thepoint of impact of the positive ions with the metal surface. Thenegative ions are emitted to travel in a circular path from the point oftheir origin until they strike a conductive surface and discharge. Theradius of curvature of the path of travel of such ions is a function oftheir initial energy, and the direction of travel is invariably counterto the direction of curvilinear travel of the positive ions responsiblefor their formation. If such negative ions impinge upon a collectorelectrode oriented to receive positive ions they K will develop at thecollector electrode a spurious signal which, if overlapping orsuperimposed on the signal produced by discharge of positive ions, willlead to erroneous interpretation of the net discharge signal.

In usual practice the beams of positive ions are selectively focused onthe collector electrode through an aperture or resolving slit in abarrier electrode. Such a resolving slit provides a high degree ofselectivity so that adjacent ion masses, i. e. ion beams not in directfocus on the resolving slit, will discharge at the barrier electrode oron the metal surfaces of the confining closure adjacent the barrierelectrode. Because of the curvilinear path of travel of the negativeions they do not generally approach the resolving slit in aperpendicular plane but rather at an angle determined by the point oforigin and the radius of the curvature of the ion. Accordingly,

I have provided improvements in mass spectromv,

eters adapted to take advantage of the above described characteristicsof negative ions to selectively trap and collect these ions withoutinterference with the travel of positive ion beams or with thecollection of positive ions.

The invention contemplates in a mass spectrometer comprising an ionsource, an analyzer tube in which ions are segregated into beams inaccordance with their specific mass, a collector electrode upon whichion beams are selectively discharged and resolving means for selectivelypassing a given ion beam to the collector, the improvement comprisingauxiliary electrode means interposed ahead of the collector electrodeoutside the path of positive ions and so arranged as to cause collectionof negative ions and thereby prevent their access to the collectorelectrode.

The invention also contemplates the entirely new concept of removingnegative ions as an infiuence in the measurement of positive ions andinvolves the method of selectively trapping the negative ions at a pointor points outside the path of positive ion flow to a collectorelectrode, such removal being conveniently accomplished in terms ofapparatus in the manner above described, but being not so limited. Theprovision of auxiliary collecting means is the presently preferredmethod of carrying out the selective removal of negative ions, but sinceremoval thereof can also be accomplished by appropriate trapping fieldsthe invention is not limited to the specific apparatus illustratedv andin general contemplates the method of selectively removing such negativeions regardless of the apparatus employed for such purpose.

The invention will be more clearly understood with reference to thefollowing detailed description thereof taken in conjunction With theaccompanying drawing in which:

Fig. 1 is a reproduction of a recording trace representing the dischargecurrent at a collector electrode when negative ions are not selectivelyremoved in accordance with the invention;.

Fig. 2 is a schematic diagram of a conventional mass spectrometerprovided with selective negative ion collecting means in accordance withone embodiment of the invention;

Fig. 3 is a diagrammatic section view of the collection system of a massspectrometer of the type shown in Fig. 2 illustrating an alternativeembodiment of the invention;

Fig. 4 is a view similar to Fig. 3 illustrating yet another alternativeembodiment of the invention; and

Fig. 5 is yet another view similar to Fig. 3 illustrating a fourthalternative embodiment of the invention.

Referring to Fig. l of the drawing the recorded trace ill therereproduced includes peaks A and B typical of the mass it and i9 peaks ofa conventional analysis with negative peaks and D preceding respectivelythe positive peaks A and B. The negative peak 0 is produced as theresult of negative ions developed as mass 18 ions strike the metalsurfaces adjacent the instrument resolving slit prior to focus on theslit to form the peak A. The negative peak D is similarly formed as aconsequence of discharge of ions of mass I9 prior to focusing these ionsthrough the resolving slit to develop peak B. The trace Iii of thefigure has been considerably simplified in the spatial separation of thepeaks C and D from the positive peaks A and B. In

actual practice, however, the negative peaks often 1 overlap or areentirely obscured by the adjoining positive peaks so as to introduce anerror into the peak reading. It is for this reason that it is desirablein accordance with the invention to remove negative ions as an innuencein the discharge current developed at a collector electrode.

A typical mass spectrometer is shown diagrammatically in Fig. 2. Theinstrument cornprises an ion source 12 including a sample inlet i3,electron beam M developed at an electron gun (not shown), a so-calledrepeller electrode 15, a first apertured accelerating electrode iii, asecond apertured accelerating electrode H and a so-called ion sourcecircuit 13 adapted to supply appropriate potentials to each of the ionsource electrodes and to the electron gun. The second acceleratingelectrode I! gives access to an analyzer tube 2a which terminates at itsopposite end in a barrier electrode 22. The particular instrumentillustrated is an isotope measuring instru ment for which purpose thebarrier electrode 22 is provided with two resolving apertures 22A, 22Band two collector electrodes 2.4, 25 aligned respectively with theapertures 22A, 22B. The two collector electrodes are in turn connectedto a ratio measuring circuit 26. The details of the ion source circuitl8 and ratio measuring circuit 25 are well known and form no part of thepresent invention. The ion source, analyzer tube and collector systemare enclosed within an i envelope 28 provided with an exhaust line 29communicating with an evacuating system (not shown).

In operation, ions formed in the ion source by the electron beam M arepropelled under the influence of potentials established between theseveral electrodes in the ion source as a heterogeneous beam into andthrough the analyzer tube 20. While passing through the tube 28 the ionsare subjected to a transverse magnetic field by magnet means (not shown)under the influence of which the beam disperses into a plurality ofseparate beams each composed of ions of a different mass-to-chargeratio. Three such separate ion beams are identified as beams a, b and cin the figure. As illustrated, the beams a and b are focused through theresolving slits 22A, 2213 respectively and discharge at the collectorelectrodes 24, respectively, the ratio of the ion currents therebyproduced being measured at the ratio circuit 26. Beam c, representativeof ion beams not focused on the resolving slits, strikes the walls ofthe analyzer chamber adjacent the barrier electrode 22 and theredischarges. In this process negative ions are given Iii:

4 off traveling in circular paths represented by the dotted lines it.The direction of rotation of these ions is counter to the direction ofcurvature of the path of the beam 0. Hence, as illustrated, the negativeions tend to approach the resolving slits 22A, 22B.

In the embodiment shown in Fig. 2 I provide a pair of conductive bafiles32, 33 extending inwardly from the barrier electrode 22 and disposedadjacent the resolving slits 22A, 2213 respectively and on the sidethereof toward the center of curvature of the analyzer tube. Since thenegative ions travel in circular paths they do not approach theresolving slit in a perpendicular plane and hence can be trapped anddischarged by bafiles as illustrated projecting into the analyzer tubeand shielding the respective resolving slits.

The collection end of a conventional mass spectrometer is shownschematically in Fig. 3 including analyzer tube at, barrier electrode iiat the end of the analyzer tube, collector electrode 42, a shieldelectrode 33 substantially enclosing the collector electrode, allcontained within an envelope A l. Disposed between the barrier electrode4i and the collector electrode is a series of apertured electrodes idelectrically connected to a suppressor grid control circuit 41 whichsupplies appropriate potentials to the electrode 16 to suppress passageof ions of less than a predetermined energy. The advantages of asuppressor grid system form no part of the present invention and havebeen previously described in the patent literature. In addition tohaving a suppressor grid system, the particular instrument shown in Fig.3 differs from that shown in Fig. 2 in having only a single aperture 41Ain the barrier electrode. Such a single resolving slit is conventionalin all but special types of mass spectrometers, that is, in the ordinaryanalytical mass spectrometer not involving isotope ratio measurement oranticipation circuits.

In this instrument ion beams striking the barrier electrode 4! in theprocess of being brought into focus on the resolving slit MA will, inthe same manner as above described, develop negative ions. Again thepaths taken by these negative ions are circular and of a radiusproportional to their energy. Thus some of the negative ions will followa circular path from the point of inception on the inner surface of thebarrier electrode which carries them through the resolving slit HA andinto the influence of the field established by the suppressor grid.Since this field is such as to repel positive ions of low energy, itwill at the same time attract negative ions and unless precautions aretaken some of such negative ions will find their way through thesuppressor grid system to the collector electrode 542. To avoid thisundesirable result, and in accordance with the invention, I provide anauxiliary apertured electrode 48 interposed between the barrierelectrode 4! and the suppressor grid system it and maintained at thesame potential as the barrier electrode M. In many instances, and asillustrated, this is a ground potential. The auxiliary electrode servesto protect the barrier electrode from the influence of the fieldestablished by the suppressor grid system so that the negative ionspassing through the resolving slit is. will follow the path representedby the dotted line it insensitive to the effects of the suppressor grid.

Another form of collection system is shown in the partial view of Fig. 4which, like Fig. 3, is a we w schematic diagram of the collectionportion of a mass spectrometer. In this particular instance the massspectrometer is provided with dual collectors 50, 52 to which positiveions gain access through a resolving slit 53A in a barrier electrode 53forming the end of an analyzer tube 54. The first collector electrode 52is apertured olT-center with respect to the barrier aperture 33A to giveaccess to the collector electrode 52, different ion beams thereby beingsimultaneously discharged on the two collectors 5D and 52. Suchsimultaneous collection of different ion beams on different collectorelectrodes is used for isotope ratio measurement and also foranticipation purposes where it is desired to set the sensitivity of arecorder in accordance with the abundance of ions of a given mass priorto collection thereof on the ultimate collector electrode. Onearrangement of dual collectors has been illustrated in Fig. 2, Fig. 4merely showing a different arrangement of such collectors. In the caseof the apparatus of Fig. 4 wherein the first collector electrode 58 isemployed for anticipation purposes only, the effects of what negativeions may collect thereon are not sufficiently pronounced to interferewith the anticipation function. However, negative ions formed on theanalyzer tube or formed at the collector 59 itself may very well passthrough the aperture 56A in the collector to gain access to thecollector 52. To prevent this I interpose an electrode trap 56 betweenthe collector 5G and the collector 52, the trap comprising a rectangularelectrode system apertured at opposite sides to give access fromaperture 53A to the collector 52, but of such dimensions that ionsfollowing a circular path through the aperture 59A will strike upon anddischarge on the electrode system 555 irrespective of their radius ofcurvature within the theoretical limits thereof. Collector electrodes5i} and 52 are connected toa sensing circuit 53 which, in this case,takes the form of an anticipation circuit operable to adjust thesensitivity of the sensing and recording network responsive to signalsfrom the collector 5i! and to thereupon record the corresponding signalfrom the ultimate collector 52.

A mass spectrometer substantially similar to that shown in Fig. 4 isshown in Fig. 5 and includes the same collectors 50, 52, barrierelectrode 53 in the end of analyzer tube 54 and having a resolvingaperture 53A with ion flow being through the aperture 53A to strike theanticipatory collector 5% or to pass through the aperture 50A therein tostrike the ultimate collector 52.

As in the foregoing instrument, the collector electrodes 59 and 52 areconnected to a sensing circuit 58. In this particular embodiment theelectrode trap 56, as shown in Fig. 4, is replaced by a single electrode651 apertured at 60A and having a tubular extension 66B extending intothe cup of collector electrode 52. The purpose of the extension 5GB isthe same as that of the electrode trap 5! namely to collect negativeions whose paths carry them through the apertures 50A and fitA. Theapparatus of Fig. 5 is not as highly emcient as that of Fig. 4 sincecertain negative ions are produced on the collector electrode 52 itself,and it is desirable that these'negative ions be discharged also on thecollector electrode, for which reason the collector electrodes arefrequently cup-shaped, as shown. If

' any of the negative ions produced at the collector electrode dischargeelsewhere than on the collector electrode, a faulty discharge signal isthereby induced, and since the tubular extension 60B does collect afraction of the negative ions developed at the collector electrode aswell as those developed elsewhere, a small error will remain. However,this error is smaller than that produced by uncontrolled negative ionsand the system of Fig. 5, although secondary in efficiency to that ofFig. 4, is still superior to any system in which negative ions areignored.

Principally the invention involves selective oollection of negative ionsat a point or points outside the path of positive ions focused onpositive ion collection electrodes, the auxiliary selective collectionmeans for the negative ions being oriented, bearing in mind thecurvilinear paths of such negative ions and the maximum theoreticalradius of such paths. With this consideration it is possible to arrangethe geometry of the system such as to prevent access of any substantialnumber of negative ions to the principal collector electrode. Differentorientation of such auxiliary selective collectors may be devised forvarious types of instruments without departing from the scope of thisinvention, and various refinements such as biasing such electrodes toattract the negative ions will occur to those skilled in the art.

I claim:

1. In a mass spectrometer comprising an ion source, an analyzer tube inwhich positive ions are segregated into beams in accordance with theirspecific mass, a collector electrode upon which ion beams areselectively discharged, and resolving means for selectively passing agiven ion beam to the collector, the improvement comprising auxiliaryelectrode means interposed in front of the collector electrode outsidethe path of positive ions and so arranged as to cause collection ofnegative ions and thereby prevent their access to the collectorelectrode.

2. In a mass spectrometer comprising an ion source, an analyzer tube inwhich positive ions are segregated into beams in acordance with theirspecific mass, a resolving slit at an end of the analyzer tube throughwhich an ion beam may be selectively focused, and a collector electrodedisposed to collect positive ions passing through the resolving slit,the improvement which comprises at least one auxiliary electrodedisposed adjacent the resolving slit and displaced from the path of theion beam focused on the resolving slit, the auxiliary. electrode beingadapted to induce collection of negative ions and thereby prevent accessthereof to the collector electrode.

3. In a mass spectrometer comprising an ion source, an analyzer tube inwhich positive ions are segregated into beams in accordance with theirspecific mass, a barrier electrode at an end of the analyzer tubeincluding a. resolving slit through which an ion beam may be selectivelyfocused, a collector electrode disposed to collect positive ions passingthrough the resolving slit and a suppressor grid disposed between thebarrier electrode and the collector electrode, the improvement whichcomprises an auxiliary apertured electrode disposed between the barrierelectrode and suppressor grid, and means maintaining the auxiliaryelectrode at a potential to shield the barrier electrode from theinfluence of the suppressor grid so that negative ions will not beattracted to the suppressor grid.

4. Apparatus according to claim 3 wherein the barrier electrode andauxiliary electrode are at the same potential.

5. In a mass spectrometer comprising an ion source, an analyzer tube inwhich positive ions are segregated into beams in accordance with theirspecific mass, a barrier electrode disposed at an end of the analyzertube and havin a resolving slit therein through which an ion beam may beselectively focused, and a collector electrode disposed to collect ionspassing through the resolving slit, the improvement which comprises atleast one auxiliary collection means disposed adjacent the resolvingslit and displaced from the path of the ion beam focused on theresolving slit, the auxiliary electrode being adapted to collectnegative ions and thereby prevent access thereof to the collectorelectrode.

6. Apparatus according to claim 5 wherein the auxiliary collection meanscomprising at least one bafile extending from the barrier electrode intothe analyzer tube from a point adjacent the resolving slit.

'7. Apparatus according to claim 5 wherein the auxiliary collectionmeans comprises an electrode trap, apertured to pass positive ions anddisposed between the barrier electrode and the collector electrodewhereby ions having paths diflering from the passed positive ions will,if passed through the resolving slit, impinge upon and discharge at theelectron trap.

8. In a mass spectrometer comprising an ion source, an analyzer tube inwhich ions are segregated into beams in accordance with their specificmass, a barrier electrode disposed at an end or" the analyzer tube andhaving a pair of spaced resolving slits through which ion beams may beselectively focused, a pair of collector electrodes disposedrespectively to collect ions passing through the resolving slits, theimprovement which comprises conductive baiiies extending from thebarrier electrode into the analyzer tube, one of such barriers beingdisposed adjacent each resolving slit, the conductive baffles beingadapted to collect negative ions and thereby present access thereof tothe collector electrode.

9. Apparatus according to claim 8 wherein the analyzer tube iscurvilinear and barriers are disposed on the sides of said resolvingslits towards the center of curvature of the tube.

1%. In a mass spectrometer comprising an ion source, an analyzer tube inwhich ions are segregated into beams in accordance with their specificmass, a barrier electrode disposed at an end of the analyzer tube andhaving a resolving slit through which ion beams may be selectivelyfocused, a first collector electrode disposed to collect ions passingthrough the resolving slit, the first collector electrode having anaperture adapted to pass a part of the ions traversing the resolvingslit, and a second collector electrode disposed to collect ions passingthrough said aperture, the improvement which comprises an auxiliaryelectrode disposed between the first and second collector electrodes andapertured to pass positive ions, the auxiliary electrode being adaptedto collect negative ions and thereby prevent access thereof to thecollector electrode.

11. Apparatus according to claim 10 wherein said auxiliary electrodecomprises an apertured plate electrode and a tubular extensionprojecting from the plate coaxially with the aperture and toward thesecond collector.

12. Apparatus according to claim 10 wherein said auxiliary electrodecomprises first and second apertured disks spaced apart in the directionof positive ion travel from the first to the second collector electrodesand electrically connected to each other.

13. In a mass spectrometer comprising an ion source, an analyzer tube inwhich ions of a given polarity are segregated into beams in accordancewith their specific mass, a collector electrode upon which ion beams areselectively discharged, and resolving means for selectively passing agiven ion beam to the collector, the improvement comprising auxiliaryelectrode means interposed in rout of the collector electrode outsidethe path of ions of said given polarity and so arranged as to causecollection of ions of opposite polarity than the ions of said ivenpolarity and thereby prevent access of said ions of opposite polarity tothe collector electrode.

14. In a mass spectrometer comprising an ion source, an analyzer tube inwhich ions of given polarity are segregated into beams in accordancewith their specific mass, a resolving slit at an end of the aria-yeartube through which an ion beam may be selectively focused, and acollector electrode disposed to collect ions of said given polaritypassing through the resolving slit, the improvement which comprises atleast one auxiliary electrode disposed adjacent the resolving slitdisplaced from the path of an ion beam focused on the resolving slit,the auxiliary electrode bein adapted to induce collection of ions or"opposite polarity from the ions of said given polarity and therebyprevent access of said. ions of opposite polarity to the collectorelectrode.

1 In a mass spectrometer comprising ion source, an analyzer tube whichions are segregated into beams of ions or". given polarity and inaccordance with their specific mass, a barrier electrode disposed at anend of the analyzer tube and having a pair of spaced resolving slitsthrough which ion beams may be selectively focused, a pair of collectorelectrodes disposed respectively to collect ions passing through theresolving slits, the improvement which comprises conductive bafllesextending from the barrier into the analyzer tube, one of such barriersbeing disposed adjacent each resolving slit, the conductive baillesbeing adapted to collect ions of opposite polarity from said givenpolarity and thereby prevent access of said ions of opposite polarity tothe collector electrode.

No references cited.

